One of the drawbacks often occurring in free-space links is the multipath phenomenon. In these circumstances the signal available across the receiving antenna is a combination of various delayed and/or attenuated signals. This combination of signals, in some instances, can lead to the impossibility of maintaining the link. A common remedy to this drawback is to resort to space and/or frequency diversity techniques that consist in the use of two suitably spaced antennas for reception or the use of two transmission frequencies. By suitably combining the two received signals, the above-mentioned drawbacks can be compensated.
One of the first works on baseband combiners is known from the article "Decision Directed Diversity Combiners Principles and Simulation Results" by J. A. Henriksson published in IEEE Journal On Selected Areas in Communications, Vol. SAC-5, No. 3, April 1987, which suggests to minimize the mean square error (MMSE) at the output of the combiner in order to combine at best the two received signals. It also introduces the use of the so-called "Minimum Projection Strategy" which consists in minimizing the mean square deflection between the received signal and the desired reference direction.
In the article "MMSE baseband combiner for microwave digital radio" by D' Andrea, M. Mariano, U. Mengali, A. Spalvieri, published in Conference Record, 2nd European Conference on Radio Relay System pages 235-242, April 1989, there is disclosed a combiner using the MMSE technique that provides a solution to the timing of the sampler by using a special algorithm. In both works some aspects are not investigated thoroughly. The first aspect is relative to the interaction between the combiner/sampler and the carrier recovery circuit present in the demodulator. A second aspect is relative to the actual cooperation of the combiner and of the sampler with particular regard to the capacity of convering to the optimal condition of operation.